How do you develop that Bulletproof Mindset, that mental strength to keep going when unwanted/negative things are happening?
Some people seem to never have bad days, they seem to never have negative thoughts or to ever have to deal with setbacks. But, every person you know or know of -- I don't care who they are or how successful, how confident they appear to be, or how much you think they have going their way -- deals with setbacks.
For people who have and maintain that Bulletproof Mindset, it's not that we don't ever experience negative feelings or that we don’t ever have negative thoughts.
The simple, key skill is to identify and replace those unproductive thoughts -- quickly and definitively.
That's all it is.
Identify your negative thoughts when they happen and replace them with the type of thoughts that you actually want — and do so quickly, as soon as you notice that useless thought creeping into your consciousness.
Imagine a salesperson who just missed a sale and the commission that would have come with it. What are the unproductive thoughts that could possibly follow, each one working off of the energy of the previous thought?
I'm not making money today.  
Come to think of it, it’s already Tuesday and I haven’t made any sales this week.  
If I miss another sale I might lose my job or not be able to pay my bills.
This is the avalanche of negative thought that happens when we don’t immediately identify and replace those unproductive thoughts: they sit and marinate in our minds.
My mom used to cook dinner — pot roasts, I believe they were called — in something called a crock pot. She’d put the meat in the pot around 7 AM, before everyone left for school and work, and let it cook until six o'clock at night when we would eat dinner.
No one needed to watch the pot for the food inside to marinate and become meal-ready over a ten-hour period. That’s exactly what happens when we have unchecked negative thoughts: they bubble up in our minds and drag us deeper into the mental abyss that first brought them on.
Our brains are the most dutiful servants we will ever employ. Any thought we focus on, the brain efficiently delivers supporting evidence that gives us more reasons to believe it. This concept works for anything we could possibly think up and focus on for a long enough period of time.
If you focus on money — earning it, as opposed to your lack of or need for it (which produces quite different results) — you'll eventually start finding ways to bring in more money.
Your conscious mind is focused on whatever's in front of you, or whatever you decide to think about.
Your subconscious mind, however, works off of whatever information you’ve fed to it, either through repetition or highly emotionalized thoughts and feelings. Highly emotionalized life experiences are alive and well in your subconscious mind even after you’ve consciously forgotten the experience.
Anything that your subconscious mind has picked up is occurring as a steady, whisper-level running conversation in your brain at all times — even while you sleep.
This is why young children (and even some adults) would be ill-advised to end their evenings with scary movies.
Whatever you give your subconscious mind before you sleep, your subconscious mind is going to marinate on it the whole 8, 5 or 10 hours that you're sleeping.
And it’s not just the last thing you give your subconscious at night (in addition to the first thing in the morning) — your subconscious welcomes and accepts anything you feed to it on a habitual basis.
Have you ever known someone who, no matter what’s happening around them, they always find a reason to get angry?
What about the person who always finds the positive, bright side of every situation?
Neither person needs to decide in that moment to be how they’re being. It’s their mental habits that lead to these responses; it doesn’t even matter what happens -- they always find what they're looking for because their subconscious minds are so used to seeing it.
It’s not the blessing of being positive or the curse of being negative -- it’s our mental habits that always find a way to show themselves.
We create these habits through the materials we feed ourselves.
The way we think.
The thinking of the people we surround ourselves with.
The shows we watch.
The materials we read.
The music we listen to.
Everything has an influence. If you want to see how, all you have to do is make a significant change in your habits in any of the above areas. Things may or may not become better for you, but I promise that they'll be different.  
Here's the real challenge, though: change does not happen in one day.
Just because you start feeding yourself the kind of thoughts, words and energy that you want reflected in your life today, doesn't mean you’re going to start getting those energies right back today.
If you poured some fruit juice out of a drinking glass, and then put that glass in the sink to let the water rise the cup, the residue from the juice doesn't automatically go away. The water needs some time to do its job displacing the residue from the fruit juice.
The negative energy and bad habits you've been living with won't curl up and die so easily. You have to run the tap of that clean water and positive thought and give it time to flush out the negative residue first.
The key is, you have to keep the water running so it can do its job.
Many people start running the “clean water” in their lives, but grow impatient when things don't change quickly enough for their liking. So they give up, believing that the clean water idea “doesn't work,” and go right back to the old habits that led to their seeking a solution in the first place.  
Turn on the faucet of positive energy and let it run. Let the water overflow the cup and rinse out all the negative residue that exists in your life, while also keeping in mind what happens if you turn that faucet off and let that clean water just sit: it, in time becomes stale and dirty.
Once get that faucet running, keep the positive momentum flowing. Stale water eventually becomes just as dirty as the residue it initially pushed out. You have to keep filling the cup, always.
How to get started with all of this: Become more aware of your thoughts.
Notice the way you’re thinking.
As soon as a negative thought enters your mind, identify and replace it with a positive thought. You’re able to do this because of the army of positive material -- books, videos, podcasts -- you've been feeding yourself every day. They'll be right there in your subconscious, waiting to be employed.

#WorkOnYourGame

Also See
#888: How To Stay Positive, Even When Things Are Going Bad
#668: Never Allow The Negatives To Outweigh The Positives
#486: Look Alive!! Bringing Positive, Can-Do Energy To Everything You Do
#101: Positive Thinking Is A Job, Not A Privilege